In a cellular system in the related art, antennae of a base station are usually arranged horizontally in an array form. Beams from a transmitter of the base station may merely be adjusted in a horizontal direction, and the each beam is provided with a fixed down-tilt angle in a vertical direction. Hence, various beamforming/precoding technologies may be performed on the basis of channel information in the horizontal direction. However, actually, a radio signal is transmitted in a space in a three-dimensional (3D) manner, so it is impossible to provide an optimal system performance through the fixed down-tilt angle.
For a 3D Multiple Input Multiple Output (MIMO) technique, one of its important features lies in that there are a large number of antennae at a base station side (a network side) and the antenna array is provided with a two-dimensional (2D) antenna structure. For example, there may exist 8, 16, 32 or 64 antennae.
Along with the development of the antenna technology, an active antenna capable of controlling each element independently has currently emerged. Through this design, in the antenna array, the antennae arranged horizontally in a 2D manner may be replaced with the antennae arranged horizontally and vertically in a 3D manner. Correspondingly, it is possible to dynamically adjust the beams in the vertical direction.
For a Frequency Division Duplexing (FDD) system, the 3D beamforming/precoding operation needs to be performed on the basis of CSI reported by a User Equipment (UE). As a possible way, the CSI may be reported on the basis of a codebook, as that adopted by a Long Term Evolution (LTE) Release 8 system.
In order to feed back the CSI using the 3D MIMO technique, in the related art, a plurality of Channel State Information Reference Signal (CSI-RS) resources may be configured for an evolved Node B (eNB) in a horizontal dimension, and pilot resources may be provided with different vertical beam-forming matrices at an eNB end. Then, the UE may measure each pilot resource in the horizontal dimension and report the CSI. Actually, in this scheme, the CSI feedback mode of a beamforming vector in the vertical dimension is combined with the CSI feedback mode in the horizontal dimension. However, there exist the following defects in this scheme.
1. There exists a very high CSI-RS resource overhead. For example, in the case that the eNB is provided with 8 beamforming vectors in the vertical dimension, 8 pilot resources need to be configured for the eNB in the horizontal direction, resulting a huge overhead for the system.
2. In order to enable eNB to acquire beam information in the vertical dimension, the UE needs to feed back signals measured on the one or more pilot resources, which results in a relatively large uplink feedback overhead.
3. The UE needs to be provided with a strong ability of processing the plurality of pilot resources in the vertical dimension (i.e., a plurality of CSI progresses), so the power consumption may increase and the UE design may be very complex, which is thus adverse to the spread of the 3D MIMO technique.
In a word, for the scheme for acquiring the CSI using the 3D MIMO technique in the related art, it is necessary to configure the pilot resources in various dimensions for the UE, resulting in a huge resource overhead. In addition, it is difficult to process the CSI progresses at a UE side, so the implementation of the scheme may be not easy.